1. Field of the Invention
The present invention relates to semiconductor processing and, in particular, concerns a method of conditioning and measuring pads used in planarizing surfaces of a wafer using chemical-mechanical polishing.
2. Description of the Related Art
Chemical-mechanical polishing (CMP) is a technique whereby surfaces are planarized by the simultaneous application of both an etching and a polishing process. Semiconductor wafers are often globally planarized using CMP processing. In a typical CMP process, the semiconductor wafer is placed on a carriage, and a pad is positioned over the wafer to contact the upper surface of the semiconductor wafer. The carriage and the pad are further rotated in opposite directions, and a slurry containing an etchant and abrasive particles flows between the upper surface of the semiconductor wafer and the pad. The combination of the mechanical polishing of the pad and the chemical etching action involved in this process serves to remove exposed surfaces of the wafer thereby planarizing the upper surface of the semiconductor wafer.
In one implementation, the CMP process is used for demascene processing in which excess layers of copper compounds are removed from the semiconductor wafer surface, leaving only the necessary copper conduit. After a time, newly removed copper compounds, such as Cu(OH)2 and Cu(OH), clog and contaminate the CMP pad, thereby degrading its planarization effectiveness. More specifically, as the CMP pad is used, contaminants build up within the pores or grooves of the CMP pad, thereby inhibiting an even distribution of slurry. This could cause uneven planarization of the wafer and necessary wafer materials might be removed.
To address these problems, methods have been developed to determine the level of contaminants on the CMP pad so as to indicate when the pad is no longer acceptable for use. One such method involves pouring a solvent, such as a solution of 1% nitric acid (HNO3), onto the pad whereby the solvent draws the contaminants from the pad. Then, a sample of this solvent and contaminant mixture is taken from the pad using a pipette, and this sample is tested for the level of contaminants present in the solution using spectrometer technology, such as ion coupled plasma analysis.
This method has several problems. For example, pouring the solution on the pad in an uncontrolled manner may actually wash away contaminants from the area that will ultimately serve as the collection point. As a result, the sample taken may not be a representative sample of the level of contaminants actually on the rest of the pad.
Also, as stated above, the contaminants often reside in the grooves and pores of the CMP pad, so in order to get samples of solvent that contain a representative amount of contaminants, the sample should be taken from within the grooves or pores. However, the pipette tips are generally too large to fit down into the narrow grooves on the CMP pad. As a result, the sample taken may be inaccurate, and the user will likely make an inappropriate assessment of the CMP pad cleanliness. Another problem is that the solvent may absorb into the CMP pad after it is poured. As a result, it may be difficult to collect the required amount of solvent for testing.
Hence from the foregoing, it will be appreciated that there is a need for a device and method that will simply and accurately measure the amounts of contaminants deposited on a CMP pad in order to determine its level of cleanliness.